1. Field
The present invention relates, generally, to analysis methods and apparatus for use with compositions of matter. More particularly, the invention relates to a method and apparatus for measuring the size and concentration of small particles in high purity liquids and colloidal suspensions. Most particularly, the invention relates to an apparatus and method for separating dissolved and particulate residues in a liquid to determine the size distribution and concentration of the particulate species (i.e. particles). The technology is useful, for example, for accurate measurement of low concentrations of very small (sub 50 nm) particles in high purity liquids, the measurement of particle retention by filters, and measurement of particle size distributions in colloidal suspensions. The invention is suitable for use in the semiconductor device manufacturing industry, the ink manufacturing industry, and in other fields.
2. Background Information
The present invention has utility in measurement of the concentration of small, for example, sub 50 nm, particles in high purity liquids. Small particles are a major problem for the semiconductor device manufacturing industry. Particles smaller than 50 nm can significantly reduce manufacturing yield of present day semiconductor devices. The ability to measure concentrations, especially low concentrations, of these particles is highly desired. Insofar as is known, there is no technology to meet this need.
The present invention also has utility in measurement of particle retention by filters, particularly those with pore sizes smaller than about 50 nm. Microporous membrane filters are often used to reduce particle levels in liquids for semiconductor device manufacturing. The ability of filters of this type to remove particles from the liquids is usually determined by challenging the filters with particles and measuring what comes through. Instruments capable of measuring particles of these sizes are not believed to be available.
The invention further has utility in measurement of Particle Size Distributions (PSD) in colloidal suspensions. There are numerous applications in which the size distribution of particles in colloidal suspensions is important in determining the efficacy of the suspension. Examples include slurries used in chemical mechanical planarization (CMP) of silicon wafers, as well as wafers composed of other materials, during semiconductor chip manufacturing and pigment-based inks. The PSD of CMP slurries determines the planarization rate, surface smoothness and scratch density on the wafer surface following the CMP process. All of these are important in determining the finished semiconductor device yield and performance. The size distribution of pigment inks is important in determining color development.
Historically, the first application mentioned above, measurement of concentrations of small particles (particularly those less than 50 nm in size) in high purity liquids has been addressed using single particle optical particle counters (OPCs). These instruments size and count individual particles as they pass through a laser beam. They have met the need of the semiconductor industry until recently, although they have typically been believed to have been a half step behind in development. Approximately twenty years ago the industry needed to detect roughly 500 nm particles; now they desire to measure 20-30 nm particles. The problem is that below about 300 nm the amount of light scattered by a particle is proportional to the 6th power of the diameter (DP6). Therefore, an instrument to measure 30 nm particles needs to be 1,000,000 times more sensitive than one that measures 300 nm particles. A leading company in making these counters has been Particle Measuring Systems. Their highest sensitivity in water is claimed to be 50 nm, but it is believed to be closer to 60-70 nm. Claimed sensitivity in chemicals is 65 nm. Particle Measurement Systems had a counter with a claimed sensitivity of 30 nm counter on the market at one time, but it is no longer available. Other companies that make counters of this type are RION, Horiba, Particle Sizing Systems, and Hach Ultra.
Very small particles (typically smaller than 10 nm) in liquids have also been analyzed using a combination of electrospray and mass spectroscopy. Electrospray is used to generate small droplets by subjecting the liquid to a high electric field. The liquid must be moderately conductive and the droplets become highly charged during formation. High purity liquids typically have low conductivity making the formation of small droplets difficult. Also, the high charge on the particles can result in particle agglomeration and may cause other changes in particle properties. The agglomeration issue can be addressed by exposing the aerosol to ionizing radiation.
The second application, measurement of particle retention by filters with small pore sizes, has also been addressed using OPCs, again limited to 50 nm. Other techniques have also been used such as turbidimitry. However, these techniques can only be used at very high particle concentrations, concentrations well above those seen in high purity applications. And, filter performance at these high concentrations is not representative of performance at lower concentrations.
Filter performance has also been measured using non-volatile residue monitors (NVRM or NRM). These instruments work by forming an aerosol of the particle-laden liquid, evaporating the liquid in the aerosol and measuring the number of particles in the aerosol. The problem with this method is that any dissolved material in the liquid forms a particle when the liquid is evaporated. Hence, the instrument measures both dissolved and particulate residue. And the residue particles interfere with the particulate particle measurement.
The third application, measurement of particle size distributions in colloidal suspensions, has typically been addressed using either dynamic light scattering (DLS) or centrifugal sedimentation. Both of these methods only measure relative PSDs. They cannot determine concentrations.
For these and other reasons, a need exists for the present invention.
All US patents and patent applications, and all other published documents mentioned anywhere in this application are hereby incorporated by reference in their entirety.